In response to concerns about global environmental pollution, in recent years intensive studies have been conducted on fuel cells for use as highly-efficient, clean power sources. Among such fuel cells, a polymer electrolyte fuel cell (PEFC) shows promise for fuel cells for automobile use, by virtue of its advantages, such as operation at low temperature and high output density. A promising fuel gas for use in PEFC is a reformed gas reformed from methanol or the like. In this connection, in order to enhance efficiency, etc., a sensor capable of direct detection of a combustible gas, such as hydrogen, contained in a reformed gas is required. Since the above-described sensor is exposed to a hydrogen-rich atmosphere, it must operate at low temperature (about 100° C. or less). Japanese Patent Publication (kokoku) No. 7-31153 proposes such a low-temperature operation type sensor. This sensor is configured such that a working electrode, a counter electrode, and a reference electrode are disposed on an insulating base material while the three electrodes are unitarily covered with a gas permeable proton conductor membrane.
The sensor disclosed in the above-described patent publication uses, as a proton conductor, Nafion (registered trademark, product of DuPont), which is one type of fluorine-containing resin and is used in polymer electrolyte fuel cells.
However, when protons travel through the above-described proton conductor such as Nafion, the protons travels together with H2O molecules. Further, the quantity of H2O molecules which accompany the traveling protons changes depending on the H2O concentration of an atmosphere. Therefore, the sensor output of a gas sensor using a proton conductor such as Nafion changes with the H2O concentration. Thus, in measurement in the presence of water vapor, no known gas sensors and methods for measuring gas concentration can measure hydrogen gas concentration without problems due to the influence of water vapor.